A. Field of the Invention
The present invention relates to a process for the manufacture of glass in which a controlled reactivity quicklime component is utilized, and to improvements in producing controlled reactivity quicklimes.
B. Description of the Prior Art
Quicklime, CaO, is produced by the calcination of limestone, CaCO3, during which processing calcium carbonate is broken down into solid calcium oxide and gaseous CO2. The resulting calcium oxide readily reacts with water to form calcium hydroxide. This reaction is rapid and very exothermic. The reaction will generate on the order of 490 BTU's per pound of quicklime. Most end uses of quicklime involve the reaction of quicklime with water or water vapor at some point in the process. The resulting exothermic reaction which occurs can be undesirable. There are also several uses of quicklime in which rapid heat evolution is neither needed nor desirable due to process parameters or safety concerns. Thus, there is a need for technology that can address these concerns.
One area of interest is that of traditional glass manufacture. The lime or limestone component present in glass manufacture serves several important functions. For example, it renders the glass more insoluble so that glass can be used unrestrictedly in contact with water and chemical solutions. It also improves the mechanical properties of glass by making it less brittle and stronger. It also improves the general appearance of glass by providing a more enduring luster. Limestone and lime are the lowest-cost fluxing materials for glass. They flux the silica sand, forming chemically fused calcium silicates.
The combined tonnage of lime and limestone in glass manufacture is close to that of soda ash and is easily the third major component of glass. The proportions of lime or limestone, stated as CaO and MgO, are shown below with the percentages of silica sand, as SiO2 and soda ash, as Na2O:
SiO268-75% Na2O10-18 CaO5-14%MgO0-10%
In addition to these basic ingredients, other chemical or mineral additives are typically added to the glass batch to produce greater opacity, decolorization, heat resistance, and other diverse physical characteristics. Batches of these raw materials are then introduced into suitable high temperature reactor vessel, where the ingredients are fused at about 1650-2200° C. to a molten state. The extremely high temperature causes the components to react to form the chemically stable but complex sodium calcium magnesium silicate, of which glass is primarily composed.
In the United States, most glassmakers tend to use limestone, CaCO3, for their source of calcium. Quicklime, CaO, offers the possibility of using less energy in the glassmaking process, since it does not have to be calcined in the glass furnace. However, highly reactive quicklime can react with free moisture/waters of hydration of other batch components causing the batches to stick together. It can also build up static charge, which interferes with the accurate movement and weighing of the quicklime portion of the batch. A properly retarded quicklime will have reduced reactivity with any water from the batch. It will also have better flow characteristics which will minimize static charge problems.
There are several existing technologies for reducing the reactivity of quicklime. However, they all suffer from one or more disadvantages. A simple way to reduce the reactivity of quicklime is to “hard burn” the limestone in the kiln. This produces a product that reacts with water at a slower pace. The down side of this technique is that it consumes more energy than regular lime production and may have to be separated out in special runs and thus be produced separately from the regular lime production. Implementing this process requires a significant amount of time to set up and is thus only effective when producing a large quantity of quicklime with a particular reactivity.
Chemical additives have also been used in the past to reduce the reactivity of quicklime. For example, Applicant's prior U.S. Pat. No. 6,395,205, issued May 28, 2002, to Huege et al., describes an improved controlled reactivity quicklime for use in manufacturing aerated autoclaved concrete materials. This patent teaches that the reactivity of quicklime may be modified through the use of a “chemical modifier” including glycerols, glycols, lignosulafonates, amines and polyacrylates, metal sulfates, gypsum, sulfuric acid, phosphoric acid, carboxylates, sucrose and mixtures thereof (column 2, lines 55-59).
The addition of water and CO2 are also known to reduce the reactivity of quicklime by coating the outside of the individual particles with calcium hydroxide and/or calcium carbonate. This method, while it works, can be difficult to precisely control as far as the final product reactivity which is achieved. The addition of weak solutions of sulfuric acid, sugar and lignosulfonates also will retard the reactivity of lime, but also have production control problems and potential deleterious side effects in the end user's process. Organics such as amines, as discussed above, will also reduce reactivity, but the presence of the organic moiety may also interfere in some manner with the final use of the lime, such as by adding chemical oxygen demand (COD) to the quicklime in some cases.
A need exists, therefore, for an improved controlled reactivity quicklime which is useful in producing glass and other manufactured products and which can be utilized for other industrial purposes.
A need exists for such a controlled reactivity quicklime which does not depend upon the calcination process itself or varying the parameters of such process.
A need exists for a controlled reactivity quicklime which can be fine tuned to produce a variety of quicklime reactivities quickly and economically, even in small quantities.
A need exists for an improved process to produce a controlled reactivity quicklime having a particular reactivity for a particular end use.
A need exists for a chemical modifier to produce a controlled reactivity quicklime which can be used to pretreat the quicklime prior to use in a glass making or other process which provides the desired degree of control over reactivity.
A need exists for a chemically modified quicklime with a controlled reactivity which does not have adverse effects on the quality of the ultimate product which is produced, such as by increasing the chemical oxygen demand of the ultimate product.